Use #[rustc_paren_sugar] as a more extensible way of deciding when

paren sugar is legal.

src/liballoc/lib.rs

@@ -70,6 +70,7 @@
 #![feature(lang_items, unsafe_destructor)]
 #![feature(box_syntax)]
 #![feature(optin_builtin_traits)]
+#![feature(unboxed_closures)]
 #![allow(unknown_features)] #![feature(int_uint)]
 #![feature(core)]
 #![feature(hash)]